In modern wireless systems (e.g., LTE, WIMAX, HSPA, UMTS, CDMA, etc) base station (BS) antennas are defined both physically and logically (in LTE they are referred to as antenna ports). Logical antennas are mapped to and implemented via physical antennas (e.g., typically several physical antennas are mapped to one logical antenna). Physical antennas are generally transparent and not visible to mobile stations (MSs), while logical antennas are generally distinguishable to MSs.
When these physical antennas are combined to yield one logical antenna, identical signals corresponding to the logical antenna are transmitted over multiple physical antennas after they are pre-processed through pre-coding matrices. These signals thus transmitted combine over the air to yield one signal transmitted over a logical antenna.
Some of these signals (such as common channels including broadcast channels) need to achieve proper sector-wide (or cell-wide as it is known in the GSM/UMTS context) coverage. Achieving this coverage is often hard especially if the number of physical antennas that make up a logical antenna is small. For example, currently two physical antennas can not provide sector wide coverage for one logical antenna according to existing techniques. Furthermore, if sufficient physical antennas are used to obtain sector-wide coverage, existing techniques produce a combined over-the-air signal that fails to fully utilize the full BS power. Also it is desirable to do this without resorting to calibration capability that is needed to measure and equalize the phase differences across the physical antennas of the logical antenna port since doing so adds cost and complexity to the antenna.
FIG. 1 illustrates one of the most common ways to achieve sector-wide static beams. As shown, each transformer 100 applies a fixed gain Ai and fixed phase φi to the same signal S. Namely, the gain and phase do not change with time such that the transmission from each antenna 110 associated with a respective one of the transformers 100 remains fixed over the time of transmission. The transmissions from the multiple antennas 110 that make up an antenna port are combined over the air.
The combining can in general (in most cases, but not all e.g., not with 2 antennas) yield proper sector wide static beams with proper choice of gains and phases. This typically requires correlated antenna configuration, i.e., under most circumstances co-polarized antennas that are spaced fractions of a wavelength, apart.
However, in general such combining often suffers from some problems. For example, such combining requires a calibration capability: measurement of gains and phases across the different transmission paths and compensating for any such differences across the different paths that exist in real physical systems. This calibration capability adds cost, operational complexity and an additional part that can fail. As another example problem, the gains that need to be employed often do not permit utilization of full transmission power of each path.
Another method is to map only one physical antenna (instead of N physical antennas) to one logical antenna port for these sector-wide beams and associated common channels. In this case, instead of utilizing the N-antenna's power for these common channels only the power of the one actual antenna that is used to map to the logical antenna port will be utilized. The disadvantage is that the BS power is significantly under-utilized and wasted for these common channels.